1. Field of the Invention
The present invention relates, for example, to a liquid crystal display device wherein a liquid crystal composition comprising a liquid crystal and a polymerizable compound that can be polymerized by active energy rays or a combination of the active energy rays and heat, is disposed between two substrates, and then, the polymerizable compound is polymerized with or without applying voltage to the liquid crystal so as to regulate the alignment direction of liquid crystal molecules.
2. Description of the Related Art
An active matrix liquid crystal display formed of TN mode, which was once in the mainstream of the liquid crystal display, has a weak point of a narrow viewing angle characteristic. At present, the techniques termed MVA (multi-domain vertical alignment) mode and IPS (in-plane switching) mode are adopted for wide-viewing-angle liquid crystal panels.
In the IPS mode, liquid crystal molecules are switched in a horizontal plane by a comb electrode. However, since the comb electrode markedly reduces aperture ratio, a robust backlight is required.
In the MVA mode, the liquid crystal molecules are aligned vertically to the substrates, and the alignment of the liquid crystal molecules is regulated by protrusions or slits (cutouts) provided on a transparent electrode (formed of, for example, ITO: indium tin oxide). Generally speaking, in the case of wide slits, the liquid crystal molecules are aligned along the direction orthogonal to a slit, while the liquid crystal molecules are aligned along the direction parallel with a slit in the case of narrow slits (fine-structured slits).
In the present MVA mode, for the purpose of obtaining a wide viewing angle, protrusions or ITO slits with complicated shapes are disposed, so that the liquid crystal molecules incline in four directions, 45°, 135°, 225° and 315°, against bus lines for signal electrodes and scanning electrodes when voltage is applied. With this technique, when voltage is applied, the liquid crystal molecules are stabilized by inclining in the four directions in parallel with the fine slits, thereby multi-domain alignment can be achieved.
In the invention disclosed in Japanese Unexamined Patent Application Publication No. 2003-149647, there has been developed a technique with a structure as shown in FIG. 2 having no protrusions in which liquid crystal molecules incline in four directions by the ITO slits when voltage is applied. At this time, by injecting a liquid crystal composition comprising a polymerizable compound between substrates, and memorizing the falling directions of the liquid crystal molecules by polymerizing the polymerizable compound while voltage is applied thereto, it is possible to shorten a response time when the falling direction of the liquid crystal molecules is propagated toward the center of a pixel, affected by the fact that the falling directions of the liquid crystal molecules are determined by the electric field at an ITO end, thereby improving the display switching characteristic.
However, even if such a complicated disposition is adopted, irregularity in the alignment of the liquid crystal molecules occurs in the vicinity of the gap located between a pixel electrode and at least either a signal electrode or a scanning electrode, when viewed from a direction substantially normal to the display screen of a liquid crystal display device. The reason for this is as follows: a side of the pixel electrode facing at least either one of the signal electrodes and the scanning electrodes is disposed in parallel with the signal electrode or the scanning electrode concerned; since the liquid crystal molecules have a property of being apt to tilt toward an electrode, the falling direction of the liquid crystal molecules in the vicinity of the above-mentioned side (end portion of the pixel electrode) becomes perpendicular to at least either one of the signal electrodes and the scanning electrodes; and, as a result, an alignment state is not stabilized in the vicinity of the pixel electrode, and the disclination is produced, which causes a varied display state.
To cope with this problem, there has been proposed a method in which auxiliary protrusions are disposed for controlling the alignment, and the portions where the alignment varies is shielded by a light-shielding layer (black matrix). However, there arises another problem of a smaller aperture ratio.
Such lowering in the substantial aperture ratio caused by the protrusions or the slits provided in the MVA mode is not so large as that produced by the comb electrode in IPS. On the other hand, as compared with the TN mode, there is a defect of a smaller light transmittance of a liquid crystal panel (display panel) in the MVA mode. As a result, in the present circumstances, the MVA mode has not been applied to a notebook personal computer which requires smaller power consumption.
Furthermore, regarding the technique for polymerizing a polymerizable compound in a state of voltage application after a liquid crystal composition comprising the polymerizable compound is injected between substrates, when the liquid crystal panel is driven for a long time period with a pattern as shown in FIG. 1A, the problem of a display defect referred to as ‘image sticking’ as shown in FIG. 1B is produced. The image sticking is a phenomenon that a tile pattern as shown in FIG. 1B remains on the display, when a black-and-white checkered pattern as shown in FIG. 1A is displayed on the display area of a liquid crystal panel for a long time, followed by the whole area being set to a certain halftone display.
Here, an image sticking rate is defined as follows:                Image sticking rate α=(β−γ)/γ)×100(%)        β=luminance of the white display area after long-time display        γ=luminance of the black display area after long-time display        
This is a phenomenon that while a liquid crystal display device is being used for a long time, the pre-tilt angle is changed in a voltage application section (at the time white is displayed in a normally black mode), which cannot easily be restored. It is considered that this pre-tilt angle change is produced by a change of the degree in regulating the liquid crystal alignment, as a result of the polymerizable compound in the liquid crystal layer that has remained after polymerization being further polymerized by the backlight at the long time operation.
In a general use condition, it is sufficiently tolerable if the image sticking rate is 2% or less after the elapse of two days. In particular, when a special usage or a special use condition is taken into account in which the same image display is continued for a long time period, no problem may occur in practical usage if the image sticking rate of the liquid crystal panel is 0% for one month. Accordingly, as a whole, an image sticking rate of 0% for one month is preferred.
Furthermore, regarding the injection of a liquid crystal or a liquid crystal composition into a liquid crystal layer, there is a problem that a truncated-V shaped abnormal display portion appears on the opposite side of a liquid crystal injection inlet, particularly when gray is displayed. This is illustrated in FIG. 12, a schematic plan view of the liquid crystal panel. Numeral 122 is a seal wall surrounding the liquid crystal layer, and numeral 123 is a liquid crystal injection inlet for injecting and sealing the liquid crystal composition, etc. into this liquid crystal layer. The truncated-V shaped abnormal display portion 121 appears on the opposite side of the liquid crystal injection inlet.
One reason for this is considered to be that the liquid crystal or the liquid crystal composition having been injected collects contaminants derived from the seal wall. That is, it is considered that the liquid crystal or the liquid crystal composition that has collected the contaminants rebounds after reaching the seal wall side opposite to the injection inlet, and the contaminants are concentrated in this rebound portion, forming the abnormal display portion. FIG. 13 is a schematic plan view of a liquid crystal panel which schematically illustrates, by means of arrows, how the rebounding of the liquid crystal or the liquid crystal composition which has collected the contaminants occurs.